U.S. President Barack Obama won strong backing from key Republican leaders Tuesday for military strikes against Syria, as Washington dramatically closed ranks to send a message to President Bashar Assad that chemical weapons must not be used.

Obama told congressional leaders that Assad must be held to account, noting that he was confident he will win "prompt" Senate and House votes on authorizing military action which he wants next week, and pledged to upgrade U.S. help to the Syrian opposition over time.

The U.S. president warned the leadership of both parties in Congress and the bipartisan leadership of key security committees that Assad had violated a vital international norm by using chemical weapons.

"That poses a serious national security threat to the United States and to the region," Obama said.

"And as a consequence, Assad and Syria needs to be held accountable.

Obama also emphasized that the operation he envisaged in Syria was "limited" and "proportional."

"It does not involve boots on the ground. This is not Iraq and this is not Afghanistan."

Asked whether he was confident that he would win votes likely next week in Congress, Obama replied at the White House meeting: "I am."

Amid doubts over whether he can deliver on a huge gamble made on Saturday to call on Congress to support U.S. military action that had seemed imminent, Obama said there was some "urgency" behind his request.

"We're going to be asking for hearings and a prompt vote," Obama told the leaders around a large table in the Cabinet Room of the White House.

"I'm very appreciative that everybody here has already begun to schedule hearings and intends to take a vote as soon as all of Congress comes back early next week," Obama said.

Obama also added credence to signs that his administration may be preparing to increase support to the Syrian opposition following the chemical weapons attack that Washington blames on Assad's forces on August 21.

He promised a strategy that would "degrade Assad's capabilities -- at the same time we have a broader strategy that will allow us to upgrade the capabilities of the opposition."

Obama was also asked whether he would be prepared to negotiate the terms of a resolution authorizing military force that the White House sent to Congress on Saturday.

Some members of Congress want to narrow the authorization to ensure that the U.S. involvement in Syria is not open ended and to codify Obama's assurance that it will not result on U.S. ground forces being used.

"So long as we are accomplishing what needs to be accomplished, which is to send a clear message to Assad, degrading his capabilities to use chemical weapons, not just now but also in the future ... I'm confident that we're going to be able to come up with something that hits that mark."

Following the meeting with Obama, Republican House Speaker John Boehner said he would support the president's call for military strikes against Syria.

Boehner noted that the United States must respond to the use of chemical weapons, adding: "I am going to support the president's call for action."

"This is something that the United States as a country needs to do," Boehner said, adding that he believed his colleagues should also support Obama's request for authorization to use military force.

"We have enemies around the world that need to understand we are not going to tolerate this type of behavior," Boehner told reporters.

"We also have allies around the world and allies in the region who also need to know America will be there and stand up when necessary."

Moments later, another key Republican, House majority leader Eric Cantor, who is popular with the party's conservative rank and file, also backed Obama's stance.

"Assad's Syria, a state sponsor of terrorism, is the epitome of a rogue state, and it has long posed a direct threat to American interests and to our partners," Cantor said.

The House, as opposed to the Democratic-led Senate, was seen as the tougher sell for Obama.

While the backing of Republican leaders for strikes does not mean that lawmakers, weary of years of U.S. wars abroad, will back military strikes, it substantially increases the odds of a yes vote, likely as soon as next week.

Assad, in a rare interview with Western media released on Monday, warned military strikes risked setting off a wider conflict in the Middle East.

"Everyone will lose control of the situation once the powder keg explodes. Chaos and extremism will spread. There is a risk of regional war," Assad said.

More than 100,000 people have died since the rebellion to oust Assad erupted in March 2011.

As part of the White House offensive to win over skeptical lawmakers, the U.S. secretaries of state and defense were to appear before a Senate panel later Tuesday.

Amid the mounting anxiety, Israel and the United States launched a missile over the Mediterranean as part of a joint exercise.

However, the Pentagon said the test was not linked to any possible U.S. military action against Syria.

"The test was long planned to help evaluate the Arrow Ballistic Missile Defense system's ability to detect, track, and communicate information about a simulated threat to Israel," Pentagon spokesman George Little said.

France, which backs Obama in his determination to launch a military intervention in Syria, Tuesday called on Europe to unite in its response to the crisis.

"When a chemical massacre takes place, when the world is informed of it, when the evidence is delivered, when the guilty parties are known, then there must be an answer," French President Francois Hollande said.

"This answer is expected from the international community," he said.

Paris Monday released an intelligence report which said Assad's forces had carried out a "massive" chemical attack last month.

Based on military and foreign intelligence services, the report said the regime launched an attack "combining conventional means with the massive use of chemical agents" on rebel-held areas near Damascus on August 21.

France has emerged as the main U.S. ally in the Syria crisis after the British parliament last week rejected involvement in any military action.

British Prime Minister David Cameron is set to push Russia and others to back a diplomatic solution to the Syria conflict at the G20 in Saint Petersburg this week.

fully agree but obama is a lawyer and calculating...he may try to get russia and iran to back off on supporting assad and give him an exit , which he will refuse and then it will be time for the fireworks